Investigation of Nanoscale Magnetic Materials and Devices
Open Access
- Author:
- Rench, David William
- Graduate Program:
- Physics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 11, 2013
- Committee Members:
- Nitin Samarth, Dissertation Advisor/Co-Advisor
Peter E Schiffer, Committee Member
Jorge Osvaldo Sofo, Committee Member
Roman Engel Herbert, Committee Member
Nitin Samarth, Committee Chair/Co-Chair - Keywords:
- Nanotechnology
magnetic
semiconductors
spintronics
spin
ice
topological
insulator - Abstract:
- A host of fundamentally and technologically intriguing phenomena can be observed in ferromagnetic systems, ranging from Giant Magnetoresistance (GMR) to spin structures that approximate the non-zero entropy state of water ice. In this dissertation, we consider systems of self-assembled MnAs nanoclusters in a doped GaAs matrix, a magnetically-doped topological insulator material, and magnetotransport devices constructed as artificial spin ices. We performed magnetic, structural, and electronic measurements in each of the projects herein to discover unique materials properties that range from new phase diagrams to electronic structure breaking and intriguing electrical characteristics that seem to defy the symmetry of the system that manifests them. We first explore the impact of co-doping a GaAs semiconductor matrix with magnetic and non-magnetic dopant ions (Mn and Be, respectively) and forcing phase separation to occur during the sample growth stage. The result of this phase-separated co-doped growth was the identification of two distinct materials classes: Type I materials, in which the phase separation produces ferromagnetic zinc blende (Mn,Ga)As nanoclusters with a narrow distribution of small diameters within a weakly Be-doped GaAs matrix, and Type II materials, in which an abrupt mixing of large NiAs-type MnAs nanoclusters and the small (Mn,Ga)As nanoclusters occurs. These two states are shown to also have accessible intermediate states in the case of a doped substrate and buffer layer. Magnetic measurements are performed to determine the dynamics of the unmixed Type I and the mixed Type II materials. Structural characterization is done at the nanoscale in a variety of instruments to precisely determine the likely growth dynamics during sample synthesis and the resultant structures. The materials are found to be superparamagnetic with 10 K (Type I) and approximately 313 K (Type II) blocking temperatures with a strong dependence on Mn content during growth determining the type of material grown. We next discuss measurements performed on the magnetically doped topological insulator candidate material Bi2Se3:Mn. The effects of Mn doping on Bi2Se3 are explored using structural and magnetic characterization techniques and the band structure is probed using Angle-Resolved PhotoEmission Spectroscopy. The material is determined to be successfully breaking time-reversal symmetry at its surface at least partially due to its suppression of the Dirac node at the gamma point of its band structure. It also shows an interesting magnetic character wherein the bulk of the material is a low temperature ferromagnet (Tc ~ 5 K) with an in-plane easy axis and the surface exhibits ferromagnetism above 100 K with an out-of-plane easy axis. The final project described here concerns an electrically continuous network of permalloy nanowires designed as an artificial spin ice. We discuss electrical measurements of these cleanroom-fabricated devices and compare the experimental results to micromagnetic simulations. We are able to successfully model the longitudinal signals seen in experiments through our simulations and also observe transverse signals in both our computations and our experiments. The latter prove to be more difficult to model computationally but we show through our computational work that the transverse signal may be analyzed in a more simplistic way than previous works have suggested. Through insights gained in our simulations, we are able to begin understanding the influence of magnetic frustration on magnetotransport studies and are able to define a clear path forward for studying other interesting phenomena using magnetoresistance measurements as the probes.